California's top air quality regulator and the head of a major construction industry trade group announced Thursday that they have reached agreement on a plan to reduce diesel emissions from construction equipment. The agreement, if approved by the full Air Resources Board in December, would end years of dispute between the board and the Associated General Contractors of America, which has fought California's diesel emission regulations for construction equipment since they were adopted in 2007.

More than 9,000 Californians are dying prematurely every year because of the health effects of the kind of pollution emitted by diesel trucks and heavy equipment, according to a new study by the Air Resources Board, the state’s air quality regulator. The study is the first released by the state to claim that the microscopic particles emitted by engines burning diesel fuel actually cause early deaths, rather than simply being correlated with them.

The California Air Resources Board acknowledged something Tuesday that critics have been saying for months: the state vastly over-estimated the amount of diesel pollution emitted by big off-road construction vehicles. The error, contained in an ARB computer model and compounded by a recession that idled far more trucks than expected, means that the construction industry would come close to meeting state-mandated targets for reducing pollution through 2025 even if regulations designed to force firms to retire or retrofit their dirtiest trucks are repealed.

A computer model that the Air Resources Board used to justify historic restrictions on diesel emissions from off-road construction equipment may have attributed twice as much pollution to those heavy trucks as they actually produce, according to interviews with ARB staff.

California’s Air Resources Board is coming under increasing pressure from construction industry contractors seeking to roll back regulations adopted three years ago to sharply reduce the amount of diesel pollution from big off-road tractors, scrapers and earth-movers. The agency has acknowledged that it underestimated the effects of the recession on diesel emissions and is also studying claims that assumptions in a faulty computer model further inflated estimates of pollution caused by the vehicles.

A new study suggests that living close to busy freeways is related to your chance of getting heart disease. The study found a statistical correlation between exposure to diesel particulate matter — the exhaust from big trucks — and the thickening of plaque on the arteries, a pre-cursor of heart disease.